Chemistry CE questions

1.
The neutralization of the acid and alkali is:
H+(aq) + OH-(aq) → H2O(l) ...... (1)

The H+(aq) and OH-(aq) are formed by ionization of CH3COOH(aq) and NH3(aq) in water. Note that the ionization of NH3(aq) need the participation of H2O(l).
NH3(aq) + H2O(aq) ≒ NH4+(aq) + OH-(aq) ...... (2)
CH3COOH(aq) ≒ CH3COO-(aq) + H+(aq) ...... (3)

To write the overall equation, add the above three equations and cancel the H+(aq) and OH-(aq) on both sides.
CH3COOH(aq) + NH3(aq) + H2O(l) → CH3COO-(aq) + NH4+(aq) + H2O(l)

The water formed in neutralization is cancelled by the water needed in ionization of NH3. The overall equation than becomes:
CH3COOH(aq) + NH3(aq) → CH3COO-(aq) + NH4+(aq)

The ionic equation is thus:
CH3COOH(aq) + NH3(aq) → CH3COONH4(aq)

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2.
The answer is B.

Rust is Fe2O3•nH2O, which is a hydrated metal oxide.

NaOH(aq) (A) and soapless detergent (D) cannot dissolve rust.

Dilute HCl can dissolve metal oxides, such as rust. It is a common property of dilute strong acid.
Fe2O3•nH2O(s) + 6H+(aq) → 2Fe3+(aq) + (n + 3)H2O(l)

Using concentrated H2SO4 to remove rust is not a good practice. This is because the degree of ionization of concentrated sulphuric acid is low, and concentrated sulphuric acid is very corrosive.

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3.
A denser gas would have a higher relative molecular mass. The average relative molecular mass of air is approximately 29.

A gas with relative molecular mass higher than 29 is denser than air. For example, the relative molecular mass of SO2 is 64.1 (> 29), and thus SO2 is denser than air.

A gas with relative molecular mass lower than 29 is less dense than air. For example, the relative molecular mass of NH3 is 17 (< 29), and thus NH3 is less dense than air.
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